A ring topology is a standard topology used in current transport network design. In this topology multiple transport nodes are interconnected to each other in the shape of a ring. As show in FIG. 1, a ring topology comprises two or more ring nodes 10-14. Each node in the ring has exactly two neighbours and the ring nodes 10-14 may be connected to other nodes in order to support traffic in surging into or abstraction from the ring topology.
Ring topologies are used with various transport technologies, by way of example optical transport networks (OTN) or electrical transport networks based on SDH (Synchronous Digital Hierarchy), ATM (Asynchronous Transfer Mode) or Ethernet standards.
Furthermore, a split router architecture is known, a concept which is currently being discussed and under development in various groups, for example in the Forwarding and Control Entity Separation (ForCes) Working Group in IETF (http://datatracker.ietf.org/wg/forces/), the group developing the OpenFlow Protocol, OpenFlow Switch Specification, Version 1.1.0, http://www.openflow.org/ or the recently created Open Network Foundation group, Open Network Foundation website, http://www.opennetworkfoundation.org/.
The split router architecture proposes to split a common router in two elements: a control element responsible for managing the routing protocol and the connectivity of the data plane. A control element controls the data plane connectivity through a forwarding element. The forwarding element is responsible for forwarding traffic in the data plane and establishes connectivity to a neighbour node based on the instructions received from the control element.
Ethernet is a widely used transport standard that specifies the physical transport layer and part of the data link layer, for example addressing. A ring topology using the Ethernet standard causes some complications. Ethernet and partly protocols above Ethernet provide automatic data path detection and selection. Those protocols must ensure that data are not sent in a loop. Various protocols are proposed to provide loop detection and loop prevention, for example the Spanning Tree Protocol, STP, and improved variants of this protocol like Rapid Spinning Tree Protocol, rSTP. These protocols provide a slow failure detection and failure handling in case a link breaks or a node fails. This failure detection and handling is in the order of seconds, this slow handling is not comparable with fail-over times achieved with the SDH technology where the failure detection handling is in the order of 50 milliseconds. A new procedure was developed in ITU-T to improve the fail-over time for Ethernet ring topology: the Ethernet ring protection switching, ITU-I G.8032/Y.1344, Ethernet ring protection switching, http://www.itu.int/rec/T-REC-G.8032-201003-I.
The specification proposes the ring automatic protection switching (R-APS) protocol to manage the connectivity and node availability in the Ethernet ring. Further functionality defined in the ITU-T recommendation “OAM functions and mechanisms for Ethernet based networks”, ITU-T Y.1731, OAM functions and mechanisms for Ethernet based networks, http://www.itu.int/rec/T-REC-Y.1731-200802-I is used to monitor the availability of links immediately connected to a node.
While OAM (Operations Administration and Maintenance) functions are used in each node to monitor the availability of the directly connected links, the R-APS protocol is used to exchange this information between all nodes in the ring. Finally, each node receives an overview of the availability of links and nodes in the ring. In case of failure, independent decisions are taken in each node to find an alternative route for the traffic bypassing the failed link or node. The concept of the Ethernet protocols used to prevent Ethernet loops is shown in FIG. 2 and a closed dedicated link for data traffic. This applies for STP, its variants and R-APS. In G.8032 the closed link is called ring protection link, RPL. Ethernet OAM traffic can still path through the RPL to monitor the link availability but other traffic is prohibited. In the embodiment shown in FIG. 2, the link between ring nodes 10 and 11 is closed and thus plays the role of the ring protection link in the example shown in FIG. 2.
As a consequence, the traffic cannot take the shortest path in all cases. By way of example, traffic from ring node 10 to ring node 11 has to path through 14, 13 and 12 as the direct connection to ring node 11 is closed. The ring protection link impacts the overall transport capacity that can be achieved in the Ethernet ring topology.
It is not easy to overcome this situation. The approach in Ethernet based networks is to run each transport node independently from one another. Each node detects the network topology by means of specific topology detection protocols. Based on information gained, each node makes an independent decision based on a common decision model to decide on how to route traffic in a network. The final model shall ensure that the final data paths are always loop-free.